In recent years, techniques of mounting one or more cameras on a vehicle and providing images captured outside the vehicle to a driver or the like have been put to practical use (e.g., around view monitors). These cameras are subjected to calibration because the image capturing ranges of the cameras are out of alignment owing to the method of mounting the cameras. Calibration is obtaining the coordinates, depression angle, rotation angle and camera pan angle of a camera. The out-of-alignment image capturing ranges are corrected based on camera parameters obtained using the results of calibration.
Hitherto, the cameras are calibrated by, for example, fixing a grid-shaped target device to the vehicle with a fixture and maintaining the space between the vehicle and the target device in a predetermined state.
Another available technique is generating, for example, a down shot image of the surroundings of the vehicle by using images captured with the cameras mounted on the vehicle. This technique performs calibration of each of the cameras mounted on the vehicle and generates a down shot image. For example, this technique obtains a street projection image of the above-described target device for each of the cameras mounted on the vehicle. For the obtained street projection images, when four cameras are mounted on the vehicle, this technique searches for a combination where 32 parameters in a homography matrix are optimum. A down shot image is generated based on the parameters detected by the search.
However, the place where the target device is fixed needs to be strict when the cameras are subjected to calibration using the above-described available techniques. This places a great burden on a user. The above-described available techniques also take a great processing time in deriving the parameters.
The followings are reference documents.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2001-285681    [Patent Document 2] Japanese Laid-open Patent Publication No. 2008-187566